What's the benefit of being able to move a different piece than the one you marked off?
if you don’t have the piece, you can’t mark/use the square.
if you’re making a beeline to the “checkmate” square, your opponent Can use this to slow you down. you can use the special move to continue uninterrupted.
if you’re trying to capture an opponent’s piece, this special move lets you do it, even if you don’t have the proper piece to use.
hope that makes sense!